Method for producing crystalline complex perovskite compounds

ABSTRACT

A method for producing crystalline complex perovskite compounds from starting compounds of at least one metal such as Ca, Ba or Sr, at least one metal such as Mg, Zn, Ni or Co and at least one metal such as Ta or Ni. The respective compounds are selected from oxides, hydroxides, hydrous oxides, and water-soluble organic compounds and inorganic salts of the respective metals. The water-soluble organic compounds and inorganic salts are capable of yielding metal oxides, hydroxides and hydrous oxides by hydrolysis. These starting compounds are heated at certain temperatures in an alkaline aqueous solution to obtain the perovskite compound at significantly low reaction temperatures of at least 95° C. Prior to the heating, the starting compounds may be treated in the alkaline aqueous solution at temperatures lower than 95° C. to form a uniformly mixed amorphous phase of the compounds and then heated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to crystalline complex perovskite compounds whichare useful as a material for capacitors or other ceramic materials andmore particularly, to a method for producing such complex perovskitecompounds.

2. Description of the Prior Art

For the manufacture of oxide ceramic powders, usual practice is to mixmetal oxides, carbonates and the like in a suitable milling device suchas a ball mill and calcine the mixture at suitable temperatures. Theresultant powder is, however, disadvantageous in that it is large andirregular in size, so that sinterability is not good; because of thenon-uniformity of the composition, the characteristics of the powder arenot consistent; and since the ingredients are mechanically mixed andmilled, for example, by a bal mill, incorporation of impurities isinevitable.

In order to avoid the above disadvantages, various preparation processesusing starting powders have been proposed. One such process is a processof hydrolyzing metal alkoxides. This process makes use of the reactionbetween a metal alkoxide of the formula, M(OR)n, in which M is a metalatom having a balance of n, and R represents an alkyl group, and waterthereby forming a metal oxide or hydroxide and an alcohol. For instance,BaTiO₃ having a perovskite structure can be prepared by dissolvingbarium alkoxide and titanium alkoxide at a molar ratio of 1:1 in anorganic solvent and dropping the solution into water for hydrolysis.This is described, for example, in Japanese Laid-open patent applicationNo. 57-82119. The powder obtained by the above process has been alreadycrystallized at a low powder formation temperature of 70° to 80° C.Further, the powder has a fine size and good sinterability, and issubstantially free of any impurities as will occur at the time of mixingin a ball mill.

Oxides which contain two or more metals and can be prepared as acrystalline powder by similar processes as described above include, forexample, perovskite compounds or solid solutions thereof such as SrTiO₃,Ba(Ti_(1-x) Zr_(x))O₃, BaZrO₃, (Ba_(1-x) Sr_(x))TiO₃ and the like asdescribed in Japanese Laid-open patent application No. 58-2220; ferritecompounds such as MnFe₂ O₄, (Mn_(1-x) Zn_(x))Fe₂ O₄, NiFe₂ O₄ and thelike as described in Japanese Laid-open Patent Application No. 56-26726;germanic acid salts such as SrGeO₃, PbGeO₃, ZnGeO₃ and the like asdescribed in Japanese Laid-open patent application No. 58-199717; andother oxides such as PbWO₄, SrAs₂ O₆ and the like.

However, the hydrolysis processes of metal alkoxides have a seriousproblem that starting metal alkoxides are very expensive.

Other processes have also been proposed to obtain a fine powder byreaction of two or more metal salts or hydrolyzates in a stronglyalkaline aqueous solution. These processes are useful in preparation ofcrystalline powders of simple perovskite compounds or solid solutionsthereof including, for example, BaTiO₃ (Journal of the IndustrialChemistry, Vol. 71, No. 1 and Japanese Laid-open patent application No.59-39716), Ba(Ti_(1-x) Zr_(x))O₃ and (Ba_(1-x) Sr_(x))TiO₃ (JapaneseLaid-open patent application No. 60-10303), and PbTiO₃ (JapaneseLaid-open patent application No. 61-158821).

For the preparation of complex perovskite compounds having a morecomplicated crystalline structure and represented, for example, by thefollowing formula,

    A(B.sub.x, C.sub.y)O.sub.3

in which A, B and C are, respectively, metal atoms having valences of p,q and r, and x+y=1 provided that q≠r and xq+yr=6-p, the above-describedknown processes or techniques are not satisfactory. For instance, withthe hdyrolysis method of metal alkoxides, an amorphous powder is formed.According to the process using a metal salt or its hydrolyzate, onlyfour compounds indicated above are now prepared, but crystalline complexperovskite compounds have not been produced by the latter process. Eventhough the individual compounds of the solid solutions are taken intoaccount, only four types of compounds including BaTiO₃, BaZrO₃, SrTiO₃and PbTiO₃ are prepared.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method forproducing crystalline complex perovskite compounds in the form of a finepowder by a simple and inexpensive manner.

It is another object of the invention to provide a method for producingcrystalline complex perovskite compounds in which the perovskitecompounds are obtained at low reaction temperatures.

The above objects can be achieved, according to the invention, by amethod of producing a crystalline complex perovskite compound of theformula, A(B_(1/3) C_(2/3))O₃, in which A represents at least onedivalent metal selected from the group consisting of Ca, Sr and Ba, Brepresents at least one divalent metal selected from the groupconsisting of Mg, Zn, Co and Ni, and C represents at least onepentavalent metal selected from the group consisting of Nb and Ta. Themethod comprises providing at predetermined mixing ratios at least onecompound of the metal A, at least one compound of the metal B and atleast one compound of the metal C, the these compounds of the metals A,B and C being selected from the group consisting of oxides, hydroxides,hydrous oxides, water-soluble inorganic salts and organic compounds ofthe metals A, B and C, respectively, provided that the water-solubleinorganic salts and organic compounds of the metals are, respectively,capable of yielding oxides, hydroxide and hydrous oxides of the metalsby hydrolysis, and reacting or treating the mixture in an alkalineaqueous solution at a temperature sufficient to cause the mixture toconvert into a crystalline complex perovskite compound. The presentinvention is characterized by the treatment of the starting compounds inan alkaline aqueous solution at certain, relatively low temperatures.

Preferably, the hydroxides, oxides or hydrous oxides obtained byhydrolysis of water-soluble metal salts or organic metal compounds areused because these compounds are more reactive and are likely to beformed as finer powders. When starting compounds of the respectivemetals are provided and are subsequently reacted in an alkaline aqueoussolution, a uniformly mixed amorphous material is initially formed. Thisamorphous material or mixture is then converted into a crystallinecomplex perovskite compound when heated at 95° C. or over. Accordingly,it is possible to form the perovskite compound from the mixture not onlyby one step where the mixture is heated in an alkaline aqueous solutionat a sufficient temperature, but also by two steps including a firststep of treating or interacting the mixture in an alkaline aqueoussolution to form an amorphous phase of the ingredients and a second stepof heating the mixture at a temperature sufficient to cause the mixtureto be converted into a crystalline complex perovskite compound. Thisheating step should be effected in coexistence of or in the alkalineaqueous solution. The term "coexistence" is intended to mean that theamorphous phase is at least wetted completely with the alkalinesolution.

DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION

According to the method of the invention, there is obtained acrystalline complex perovskite compound of the formula, A(B_(1/3)C_(2/3))O₃, in which A represents at least one divalent metal selectedfrom Ca, Sr and Ba, B represents at least one divalent metal selectedfrom Mg, Zn, Co and Ni, and C represent at least one pentavalent metalselected from Nb and Ta.

The three types of metals represented by A, B and C should beessentially contained in final perovskite compounds of the invention.The respective metal components A, B and C may each consist of one ormore metals selected from the defined group.

In the first step of the method of the invention, starting oxides,hydroxides, hydrous oxides, and watersoluble inorganic salts and organiccompounds of the respective metals are provided. If oxides, hydroxidesand hydrous oxides are used, these materials should be as fine fine aspossible as provided. The oxides, hydroxides or hydrous oxides obtainedfrom the water-soluble inorganic salts or organic compounds arepreferred because they are fine in size and reactive although it is notalways necessary to use those starting compounds obtained by hydrolysisof water-soluble metal salts or organic metal compounds. However, if thestarting materials are not soluble in water and have a size over 1micrometer, they are unlikely to readily form a uniformly mixedamorphous phase of the starting compounds. When such a mixture issubjected to reaction or treatment in an alkaline aqueous solution, theconversion reaction does not smoothly proceed to a satisfactory extent.As a result, it takes inconveniently a long time for the conversionreaction while leaving part of the starting materials unreacted. Eventhough the size is below 1 micrometer, the powders which have once beensubjected to thermal treatment at high temperatures are poor inreactivity. This is why oxides, hydroxides or hydrous oxides obtainedfrom metal salts or organic metal compounds by hydrolysis are preferablyused. In fact, the oxides, hydroxides or hydrous oxides obtained fromthese soluble metal salts or organic metal compounds are more reactiveand finer in size than available solid oxides, hydroxides or hydrousoxides of the defined metals. These water-soluble metal salts or organicmetal compounds may be used as they are or after conversion intocorresponding oxides, hydroxides or hydrous oxide. If the salts ororganic metal compounds are added to the reaction system, they arereadily converted into corresponding oxides, hydroxides or hydrousoxides in the system since the system is aqueous and alkaline in nature.

The salts useful for these purposes may be water-soluble inorganic saltssuch as sulfate, halides including chloride, bromide, iodide andfluoride, phosphate, nitrate, and the like. If it is desirable toseparately obtain oxides, hydroxides or hydrous oxides from these salts,a metal salt is dissolved in water or an acidic solution and neutralizedwith an alkali as will be particularly described in the examples.

The organic metal compounds include, for example, alkoxides such asmethoxide, ethoxide, propoxide and the like, carboxylates such asacetate, propoxide and the like, and acetylacetonates, and the like. Inorder to separately obtain oxides, hydroxides or hydrous oxides fromthese organic metal compounds, these compounds are dissovled in asuitable solvent and the resultant solution is added to water.Alternatively, the compound may be dissolved in water and neutralizedwith an alkali.

Irrespective of the origins of the starting oxide, hydroxide or hydrousoxide powders, they should preferably have a size not larger than 1micrometer.

These starting materials should be provided at predetermined mixingratios so that a final perovskite compound has intended ratios of themetals, A, B and C.

In the second step, the starting materials are subjected to conversionreaction into a crystalline complex perovskite compound. This conversionreaction is effected in an alkaline solution at a temperature sufficientto cause the starting materials into the perovskite compound. in orderto make an alkaline solution, an alkaline compound such as NaOH, KOH orthe like may be used. In this connection, however, since the complexperovskite compound contains an alkaline earth metal such as Ba, it ispreferred to use a hydroxide or oxide of this alkaline earth metal to becontained in a final perovskite compound. This makes it possible toprevent impurities such as Na, K and the like from incorporation intothe perovskite compound.

In one embodiment of the invention, the conversion reaction is effectedby one step which comprises reacting or treating the mixture of oxides,hydroxides, hydrous oxides, water-soluble inorganic salts or organiccompounds of the metals A, B, and C in an alkaline aqueous solution at atemperature sufficient to cause the mixture to be converted into acrystalline complex perovskite compound.

The reacting or treating conditions are described. The conversionreaction proceeds satisfactorily in the alkaline aqueous solution. Ifthe aqueous solution used is too small in amount, the reaction does notnecessarily proceed smoothly. On the contrary, too large an amount ofthe aqueous solution undesirably requires a large amount of an alkalinesubstance. It is thus preferred from the industrial viewpoint that thestarting materials are used in an amount of 0.1 to several moles perliter of the solution when calculated as a final complex perovskitecompound.

In view of the reaction time and yield, the reaction system shouldpreferably have a pH of 10 or over. At a pH below 10, the reactionproceeds only slowly. More preferably, the pH of the reaction system is11 or over, most preferably 12 or over. The starting materials usedcontain an alkaline earth element such as Ba, Mg and the like. If thesematerials are soluble in water, the reaction system eventually becomesalkaline and the reaction may proceed without use of any additionalalkaline substances. However, this will result in formation of a finalperovskite compound having a composition different from an intendedcomposition owing to the dissolution of part of the starting powders. Toavoid this, other alkaline substances such as NaOH may be added to thereaction system but, as described before, the contamination of the finalproduct with Na would become inevitable. Accordingly, it is preferred touse an aqueous saturated solution of a starting alkaline compound as analkaline aqueous solution, or to permit part of a starting alkalinecompound to be dissolved in water and continue the reaction until thewater is gradually evaporated to dryness.

The conversion reaction is considered to proceed by two steps: one stepincludes interaction of three kinds of starting materials in an alkalineaqueous solution to form a uniformly mixed amorphous powder mixture ;and the other step includes crystallization of the amorphous powder byheating at temperatures not lower than 95° C. in the solution to form acrystalline complex perovskite compound.

The first step proceeds more fastly at higher temperatures, but thetemperature is not critical. The temperature higher than 70° C. ispreferable in order to cause the interaction efficiently. The timebefore completion of the interaction may vary depending upon not onlythe interaction temperature, but also the types and amounts of startingmaterials. The intereaction time may be in a wide range of 0.5 hours or100 hours, preferably several to several tens hours. The uniformly mixedamorphous phase obtained by the interaction is considered to be a kindof precursor in which metal--O--metal bonds may be formed along withmetal--OH bonds.

In the crystallization step, the temperature is important. When theinteraction has been completed and the resultant amorphous powder isheated under contact with the alkaline solution at a certaintemperature, the crystallization proceeds. A crystalline complexperovskite compound starts to be formed at temperatures of 95° C. orhigher as will be clearly seen from the examples. The heatingtemperature is preferably not lower than 100° C., more preferably notlower than 105° C. for ensuring a smooth crystallization procedure. Attemperatures higher than 100° C., the crystallization is generallyeffected in a closed system, such as an autoclave, under pressurebecause the reaction system is aqueous.

In one embodiment of the invention which comprises one step reactionprocedure as described before, the reaction system is heated to 95° C.or higher from commencement of the treatment in coexistence of or in analkaline aqueous solution so that the starting materials are convertedthrough an amorphous powder into crystallized by one step. This one stepprocedure is preferably effected in an autoclave or the like closedsystem so that temperatures of 95° C. or higher are conveniently used.

In another embodiment of the invention, the interaction is effected attemperatures lower than 95° C. for a sufficient time of 0.5 hours orlonger in an aqueous alkaline solution to obtain a uniformly mixedamorphous powder. Preferably, the temperature is 70° C. or higher.Subsequently, this powder is heated at 95° C. or higher in coexistenceof or in the alkaline solution. This embodiment is different from thefirst embodiment in that the amorphous powder is initially formed bytreatment in the alkaline aqueous solution at temperatures lower than95° C., at which crystallization starts to occur, and is thencrystallized at temperatures higher than 95° C. as set forth withrespect to the first embodiment. For the crystallization, the thusobtained amorphous powder may be removed from the solution but fullywetted therewith and heated at temperatures not lower than 95° C.Alternatively, the amorphous powder in the aqueous alkaline solution maybe heated at it is at temperatures not lower than 95° C. for a timesufficient for crystallization, or may be heated until the mixture isevaporated to dryness if at least one of the starting materials issoluble in water and is used to make an alkaline solution. Thisembodiment using the distinct two interaction and crystallization stepsis advantageous in that crystals of a final perovskite compound arefiner and more uniform in size.

In either embodiment, the conversion reaction is preferably effected inan atmosphere of a gas free of CO₂ so that formation of a crystallinecomplex perovskite compound is not impeded at all. This is because if Bais contained, for example, in the starting materials, it may react withCO₂ and convert into barium carbonate, which is insoluble in water.Accordingly, it is preferred to effect the reaction in an atmosphere ofa CO₂ -free gas such as N₂ or O₂, or other inert gases such as Ar.

The conversion reaction takes a relatively long time, so that thereaction should preferably be effected in a closed or substantiallyclosed system, not permitting the water in the reaction system to beevaporated by heating. For this purpose, a system using a refluxcondenser as well as an autoclave may be conveniently used to recycleonce evaporated water. In this connection, it should be noted that ifstarting alkaline materials are used to make an alkaline reaction systemwithout use of any additional alkaline substances, the water in thesystem may be purposely evaporated off by heating so as to bring a finalcomposition of the perovskite to conform to an initially intendedcomposition. The treatment or reaction may be carried out underagitation or merely under reflux.

The final perovskite product is usually obtained in an alkalinesolution. The product may be separated from the solution by any knowntechniques such as filtration, centrifugal separation and the like.

Typical and preferable examples of final crystalline complex perovskitecompounds prepared according to the method of the invention includeBa(Zn_(1/3) Ta_(2/3))O₃, Ba(Mg_(1/3) Ta_(2/3))O₃, Sr(Mg_(1/3)Nb_(2/3))O₃ and the like.

The present invention is more particularly described by way of examples.

EXAMPLE 1

Prior to preparation of a complex perovskite compound, startingcompounds were first prepared. Special grade ZnSO₄ was dissolved in purewater, to which NaOH was added in order to make a pH of approximately6.5. The resultant precipitate was separated from the solution andwashed sufficiently with pure water to obtain zinc hydroxide.Separately, TaCl₅ was dissolved in a mixture of nitric acid andhydrofluoric acid and neutralized in the same manner as with Zn toobtain an amorphous hydrolyzate of tantalum. The thus obtained powderswere each heated up to 1000° C. to determine an ignition loss, fromwhich the contents of Zn and Ta were, respectively, determined. Thesepowders and special grade Ba(OH)₂.8H₂ O were accurately weighed in suchamounts that Ba:Zn:Ta=3:1:2 by atomic ratio. The powders were mixed withan aqueous saturated solution of barium hydroxide in a flask. The flaskwas set on a water bath and heated under reflux in a stream of N₂ gas at80° C. for 5 hours. The resultant precipitate was separated by acentrifugal separator, and divided into pieces. The respective pieceswere wrapped with paper in a wet condition and dried at differenttemperatures of 30° C., 90° C., 95° C., 100° C. and 110° C. to obtainthe respective powder products. These products were each subjected toX-ray diffraction. As a result, it was found that the sample dried at30° C. consisted of an amorphous phase and barium hydroxide. The sampledried at 90° C. was entirely amorphous. Those samples dried at 95° C.and 100° C. showed peaks of a crystalline complex perovskite phase ofBa(Zn_(1/3) Ta_(2/3))O₃ although these peaks were broad. The sampledried at 110° C. was found to be crystalline Ba(Zn_(1/3) Ta_(2/3))O₃.

EXAMPLE 2

In the same manner as in Example 1, Ba(OH)₂.8H₂ O, and amorphoushydrolyzate product of tantalum and ZnO obtained by precipitation froman aqueous solution of ZnSO₄ at a pH of 10 were provided in such amountsthat Ba:Zn:Ta=3:1:2 by atomic ratio. These powders were added to 50 mlof pure water and heated under reflux in an stream of N₂ at 90° C. for 5hours. Thereafter, a reflux condenser was removed and the temperaturewas raised to 100° C., at which the content was heated under agitationuntil it was evaporated to dryness. The resultant powder was subjectedto measurement of X-ray diffraction, revealing a very broad diffractionpeak of a crystalline complex perovskite phase of Ba(Zn_(1/3)Ta_(2/3))O₃. Example 3

Ta(C₂ H₅ O)₅ was dissolved in ethanol, to which pure water was addedthereby obtaining an amorphous hydrolyzate product of tantalum. Thistantalum product, BaSO₄ and ZnSO₄ were provided in such amounts thatthat Ba:Zn:Ta=3:1:2 by atomic ratio, followed by addition to 100 ml ofpure water. Thereafter, an aqueous NaOH solution was added to themixture in an amount of 0.032 moles as NaOH, followed by heating underreflux in a stream of N₂ at the boiling point for 5 hours.

The resultant powder was dried and subjected to X-ray diffraction, fromwhich it was confirmed that a crystalline complex perovskite compound,Ba(Zn_(1/3) Ta_(2/3))O₃, was formed although the peak was very broad.

EXAMPLE 4

Similar to Example 2, Ba(OH)₂.8H₂ O, an amorphous tantalum hydrolyzateand ZnO obtained by precipitation from an aqueous solution of ZnSo₄ at apH of 10 were, respectively, provided in such amounts that thatBa:Zn:Ta=3:1:2 by atomic ratio. These powders were added to 50 ml of anaqueous saturated solution of barium hydroxide. The dispersion wasplaced in a 200 ml autoclave and subjected to reaction at 120° C. for 5hours. Thereafter, the content was cooled down to room temperature andthe resultant powder was separated by filtration and dried. This powderwas subjected to X-ray diffraction analysis, from which it was confirmedthat a crystalline complex perovskite compound was formed. The formedcrystals had a size of about 0.1 micrometer.

EXAMPLE 5

Similar to Example 1, Ba(OH)₂.8H₂ O, an amorphous hyrolyzate of tantalumoxide and MgO were, respectively, provided in such amounts thatBa:Mg:Ta=3:1:2 by atomic ratio. These powders were added to 150 ml of anaqueous saturated solution of barium hydroxide and magnesium hydroxideand placed in a flask. The above procedure was repeated, thereby givingthree flasks in total. These flasks were, respectively, set on a waterbath and heated under reflux in a stream of N₂ at 80° C., 90° C. and 95°C. for 5 hours. The resultant precipitate was separated by means of acentrifugal separator, transferred to a beaker and dried in a dryer at80° C. The powders obtained under different conditions were subjected toX-ray diffraction analysis. As a result, it was found that the powderstreated at 80° C. and 90° C. were amorphous. On the other hand, thepowder treated or reacted at 95° C. showed a diffraction peak of acrystalline complex perovskite phase of Ba(Mg_(1/3) Ta_(2/3))O₃ althoughthe peak was very broad.

EXAMPLE 6

Similar to Example 6, Ba(OH)₂.8H₂ O, MgO and an amorphous hydrolyzate oftantalum were, respectively, provided in such amounts thatBa:Mg:Ta=3:1:2 by atomic ratio. These powders were added to 50 ml ofpure water and placed in a 200 ml autoclave, followed by treatment at110° C. for 5 hours. Thereafter, the content in the autoclave wastransferred to a teflon beaker and dried at 100° C. The resultant powderwas subjected to X-ray diffraction analysis, revealing that acrystalline complex perovskite phase of Ba(Mg_(1/3) Ta_(2/3))O₃ wasformed. The size of the powder determined from the X-ray diffractionanalysis was about 0.1 micrometer.

EXAMPLE 7

Ta(C₂ H₅ O)₅ was dissolved in ethanol, to which pure water was addedthereby obtaining an hydrolyzate product of tantalum. This tantalumproduct, BaSO₄ and MgSO₄ were, respectively, provided in such amountsthat Ba:Mg:Ta=3:1:2 by atomic ratio. These powders were added to 100 mlof pure water, to which an aqueous NaOH solution was added in such anamount that the content of NaOH was 0.032 moles. The mixture was reactedby heating in a stream of N₂ under reflux at the boiling point of about100° C. for 5 hours. Subsequently, the content was cooled down to roomtemperature and the resultant powder was filtered off and dried. Thedried powder was subjected to X-ray diffraction analysis, from whichformation of a crystalline complex perovskite compound was confirmed.The crystals of the powder had a size of about 0.2 micrometers.

EXAMPLE 9

A precipitate obtained by neutralizing an acidic aqueous solution ofNbCl₅ was washed sufficiently with pure water and dried to obtain anamorphous hydrolyzate product of niobium. This niobium product, specialgrade strontium hydroxide and magnetic oxide were, respectively,provided in such amounts that Sr:Mg:Nb=3:1:2 by atomic ratio. Thesematerials were added to 150 ml of an aqueous saturated solution ofstrontium hydroxide and magnesium hydroxide and placed in an autoclave.The above procedure was repeated to obtain six samples in total. Thesesamples were reacted for 2 hours at different temperatures of 80° C.,90° C., 95° C., 100° C., 110° C. and 140° C. The resultant reactionproducts were each separated by filtration, washed with water and driedat 80° C. Each product was subjected to X-ray diffraction analysis. As aresult, it was found that the products obtained by the reaction at 80°C. and 90° C. were amorphous. The products obtained by the reaction at95° C. and 100° C. were observed to have broad diffraction peaks ofcrystalline Sr(Mg_(1/3) Nb_(2/3))O₃. The products obtained by thereaction at 110° C. and 140° C. were crystalline Sr(Mg_(1/3)Nb_(2/3))O₃.

EXAMPLE 10

Similar to Example 9, an amorphous hydrolyzate product of niobium,special grade strontium hydroxide and magnesium oxide were,respectively, provided in such amounts that that Sr:Mg:Nb=3:1:2 byatomic ratio. These materials were added to 50 ml of pure water andreacted by heating under reflux in a stream of Ar at 90° C. for 3 hours.

Thereafter, a reflux condenser was removed and the temperature of thereaction system was raised to about 100° C., followed by heating untilthe water was completely evaporated. The resultant powder was subjectedto X-ray diffraction analysis, with the results that a broad diffractionpeak of Sr(Mg_(1/3) Nb_(2/3))O₃ was observed.

EXAMPLE 11

Niobium ethoxide, special grade strontium sulfate and magnetic hydroxidewere, respectively, provided in such amounts that Sr:Mg:Nb=3:1:2 byatomic ratio. These materials were added to 200 ml of an aqueous NaOHsolution having a concentration of 1 mole and reacted in an autoclave ina stream of Ar at 110° C. for 5 hours. The resultant powder wassubjected to X-ray diffraction analysis, from which a diffraction peakof crystalline Sr(Mg_(1/3) Nb_(2/3))O₃ was observed. The crystals of thepowder had a size of about 0.1 micrometer.

What is claimed is:
 1. A method of producing a fine powder, crystallinecomplex perovskite compound of the formula

    A(B.sub.1/3 C.sub.2/3)O.sub.3,

wherein A represents at least one divalent metal selected from the groupconsisting of Ca, Sr and Ba, B represents at least one metal selectedfrom the group consisting of Mg, Zn, Co and Ni, and C represents atleast one pentavalent metal selected from the group consisting of Ta andNb, the method comprising providing a proportion of at least one memberselected from the group consisting of oxides, hydroxides, hydrousoxides, water-soluble inorganic salts and organic compounds of themetals A, at least one member selected from the group consisting ofoxides, hydroxides, hydrous oxides, water-soluble inorganic salts andorganic compounds of the metals B, and at least one member selected fromthe group consisting of oxides, hydroxides, hydrous oxides,water-soluble inorganic salts and organic compounds of the metals C,provided that the watersoluble inorganic salts and organic compounds ofthe respective metals are, respectively, capable of yielding oxides,hydroxide and hydrous oxides of the metals by hydrolysis, said memberwhich are not water soluble having a size of one micrometer or less, theproportion of said members being such as to effect formation of saidperovskite compound; and reacting the selected members in an alkalineaqueous solution at a temperature of 140° C. or lower, said temperaturebeing sufficient to cause the members to convert into the fine powder,crystalline complex perovskite compound.
 2. A method according to claim1, wherein said crystalline complex perovskite compound is Sr(Mg_(1/3)Nb_(2/3))O₃.
 3. A method according to claim 1, wherein said crystallinecomplex perovskite compound is Ba(Mg_(1/3) Ta_(2/3))O₃.
 4. A methodaccording to claim 1, wherein said crystalline complex perovskitecompound is Ba(Zn_(1/3) Ta_(2/3))O₃.
 5. A method according to claim 1,wherein said alkaline aqueous solution is a solution of a hydroxide ofthe metal A.
 6. A method according to claim 5, wherein the solution of ahydroxide of the metal A is a saturated solution.
 7. A method accordingto claim 1, wherein said alkaline aqueous solution is a solution of analkali metal hydroxide.
 8. A method according to claim 1, wherein saidalkaline aqueous solution has a pH not lower than
 12. 9. A methodaccording to claim 1, wherein the temperature is higher than 95° C. 10.A method according to claim 1, wherein the temperature is higher than110° C. and the reaction is effected in a closed system under pressure.11. A method according to claim 1, wherein the reaction is effected in asystem using a reflux condenser so that the water in the alkalineaqueous solution is recycled.
 12. A method according to claim 1, furthercomprising heating the alkaline aqueous solution to dryness when atleast one of the selected members is soluble in water and is used tomake the alkaline aqueous solution.
 13. A method according to claim 1,further comprising separating the obtained perovskite compound from thealkaline aqueous solution.
 14. A method according got claim 1, whereinthe reaction is effected in a CO₂ gas-free atmosphere.
 15. A methodaccording to claim 1, wherein the reaction is effected by a procedurewhich comprises treating the selected members in an alkaline aqueoussolution at a temperature lower than 95° C. to form a uniformly mixedamorphous powder, and heating the amorphous powder in the alkalineaqueous solution or in coexistence therewith at a temperature not lowerthan 95° C. to convert the powder into a crystalline complex perovskitecompound.
 16. A method according to claim 15, wherein the amorphouspowder is heated at a temperature higher than 110° C.
 17. A methodaccording to claim 15, wherein prior to the heating, the amorphouspowder is removed from the solution but kept wetted, and is subsequentlyheated.
 18. A method according to claim 15, wherein the amorphous powderis heated in the alkaline aqueous solution.
 19. A method according toclaim 15, wherein the amorphous powder is heated to dryness.